The present invention relates to a method for manufacturing nonwoven fabrics and apparatus for manufacturing the same. More specifically, the present invention relates to a method for manufacturing nonwoven fabrics particularly suitable for manufacturing nonwoven fabrics which are formed from extra fine fibers of which fineness is less than one denier.
A span bond method for hauling and drawing resins extruded from a nozzle by means of an air sucker is a method for manufacturing nonwoven fabrics with good productivity. In this method, generally, continuous fibers having a fineness of 1.5 denier or more are manufactured.
On the other hand, a melt blow method is employed as a method for manufacturing nonwoven fabrics of which fineness is less than one denier. According to this method, resins moved out of a nozzle are blown off by high speed heated gases blown out of gas orifices around the nozzle to obtain extra fine fibers.
In the above-described span bond method which hauls resins by the air sucker, when the resins having the fineness less than 1 denier are spun, cutting of yarns often occurs during spinning for the reasons hereinbelow, failing to achieve stabilized production.
More specifically, the molten resin extruded out of the nozzle first moves forward in substantially the same diameter as the bore diameter of the nozzle for a distance to some extent, and thereafter the resin suddenly becomes fine at a certain location and is drawn. Such a portion is called a neck. Such drawing of the molten resin extruded out of the nozzle is not carried out in the whole area of the spinning section but is rapidly carried out at the neck. Therefore, formation of finer resin becomes unstable as the ratio of fiber diameter before and behind the neck increases and as the gradient in change of section at the neck becomes severe.
A method for reducing a bore diameter of a nozzle is employed as a method for reducing the severe change of section before and behind the neck. However, this method has not been put to practical use due to the problems of processing technique of nozzles and blockage of nozzles by foreign matter. Therefore, it is difficult for the conventional span bond method to manufacture soft nonwoven fabrics formed from fibers of which finesness is less than one denier.
On the other hand, in the melt blow method, gases blown out of gas orifices have their initial speed of hundreds of meter/second but the speed thereof rapidly attenuates as the gases move away from the nozzle. Therefore, the fibers momentarily drawn and tensioned by the high speed gases are relieved from tension without being sufficiently cooled. Accordingly, the obtained fibers are small in strength. In addition, resin used are small in melting viscosity and small in molecular weight so that the resins may withstand the instantaneous high speed drawing as described above, and therefore they are originally poor in representation of strength.
For these reasons, nonwoven fabrics having a small fineness can be manufactured by the melt blow method but the obtained nonwoven fabrics is small in strength of fiber, say, 1/2, as compared with the previously described method using the air sucker. Furthermore, the fibers are not completely continuous but the length of fibers is from approximately 1 meter to several centimeters, in which is mixed a small lump of resins called a shot.